Multi-axial-mode laser effects in polarization spectroscopy

The effects of multifrequency-mode laser radiation on polarization-spectros copy signal generation are investigated by direct numerical integration of the time-dependent density-matrix equations. The numerical solution of the density-matrix equations allows us to incorporate a physically reasonable m odel. for pulsed dye-laser radiation in our analysis of the laser-resonance interaction. The laser radiation is modeled as the sum of electric fields from a finite number of modes that are assumed to have random pulse-to-puls e phases and exponentially distributed amplitudes. Calculations are perform ed for a homogeneously broadened resonance (only collisional broadening) an d for a resonance that is both collision and Doppler broadened. The effect of the multimode laser radiation on polarization-spectroscopy line shapes a nd saturation curves is investigated for different values of the laser band width and mode spacing and resonance collision and Doppler widths. The satu ration parameter for the resonance is strongly dependent on the ratio of th e laser bandwidth to the resonance collision width when the laser bandwidth is greater than the collision width. The pulse-to-pulse fluctuations in po larization-spectroscopy signal levels are found to decrease substantially f or saturating pump intensities. The inclusion of the multimode laser struct ure into our density-matrix equations represents a significant advance in m odeling the nonlinear interaction of laser radiation with atomic or molecul ar resonances. (C) 2000 Optical Society of America.